The predominant utilization of solar energy to drive the chemical reduction of atmospheric carbon dioxide is through photosynthesis by plants. This process reduces the level of the greenhouse gas in the atmosphere, and also leads to the formation of useful carbon-based fuels.
Achieving both of these goals with an industrial process is generally recognized as having great utility. This is especially true if the material inputs for such a process are inexpensive, and do not include large amounts of water. Such a hypothetical industrial process for solar-powered photochemical reduction of CO2 could open up large non-productive areas of the Earth's surface, e.g. deserts, to utilization of the large amounts of solar energy that fall upon them. Toward this goal, a great deal of effort has been applied in attempting to discover inexpensive methods for using the energy of sunlight to carry out chemical reduction of carbon dioxide, with the formation of elemental oxygen along with a carbonaceous material—either elemental carbon, or (using an additional material input such as hydrogen), some compound of carbon that stores a similar amount of energy as elemental carbon.
Carbon dioxide itself is capable of absorbing only a tiny fraction of the wavelengths of the solar spectrum that reach the Earth's surface, and essentially none of the visible or UV wavelengths that are most likely to be able to initiate photochemical reactions. Thus, a major thrust of the research in solar energy conversion has been to develop sensitizing chemicals that are capable of absorbing solar photons, thereby creating a high-energy photochemical product, which then can carry out chemical reactions with CO2 leading, ultimately, to the formation of molecules containing chemically-reduced carbon.
Known methods for photochemical reduction of CO2 have generally involved transition-metal compounds, with or without organic dye sensitizers. In a number of examples, titanium dioxide (titania) has been used as both a nanoparticle support and as a surface-active electron transfer agent, in order to photochemically reduce CO2. In several examples of this approach, cobalt phthalocyanine (CoPc) was used as a photosensitizer (Kuwabata et al., J. Chem. Soc. Chem. Comm., (1995) 829-830; Liu et al. (2007), Photochem. Photobiol. Sci. 6, 695-700). In the first example, CO2 reduction to methanol was achieved by using a non-aqueous solvent, polypropylene carbonate ester. In the second example, it was hypothesized that even in aqueous media, electron transfer could proceed directly from the CoPc photosensitizer's excited state to a weakly liganded CO2 molecule. In this case, most of the product was formic acid, which is not directly useful as a fuel but has greater value as a feedstock for chemical synthesis of larger molecules.
Both of the cited examples demonstrate the chief shortcoming of existing methods. In particular, the CoPc sensitizer is a complex molecule that requires considerable energy to produce; adsorbing it to titania nanoparticles requires additional processing; and the yields of reduced-carbon product so far remain low. These workers [Liu et al 2007] reported that even under optimal conditions, the total yield of reduced carbon was about 1 millimole (i.e. 12 mg carbon) per gram of catalyst. It is therefore unclear whether the low yield from such a photocatalyst will lead to a net storage of solar energy, or whether this conversion could ever be economical given the likely high cost of synthesizing the CoPc sensitizer. Cobalt itself is relatively common (crustal abundance of 25 ppm), with a bulk cost of the element about $44 per kg. However the CoPc sensitizer used in this particular process requires considerable additional synthesis steps, and currently has a bulk cost of over $5000 per kg.
In a more recent example utilizing a different type of sensitizer, both a ruthenium-bis-pyridyl compound and the enzyme carbon monoxide dehydrogenase were attached to titania nanoparticles in order to achieve photoreduction of carbon dioxide to carbon monoxide [Woolerton, et al., J. Am. Chem. Soc., 132, 2132-2133 (2010)]. A measure of the efficiency of such conversion systems is the turnover number of the photosensitizing agent. In this case, it was possible to achieve a turnover number of 250 μmol of CO2 per gram of TiO2 per hour. However, in this case the chemical complexity of the nanoparticles has been increased even further than with the CbPc. Here, not only is a small-molecule sensitizer required (the ruthenium complex), but also an enzyme. As in the preceding examples, the high energetic and economic costs of producing the sensitized nanoparticles are significant disadvantages. Ruthenium's crustal abundance is approximately 1 ppb; current worldwide annual production is only 30 tons. The bulk cost is $6500 per kg. Furthermore, this bulk ruthenium requires costly additional chemical processing with a bis-pyridyl moiety before it can be made into a suitable photosensitizing compound, which makes its overall cost even higher on a per-mole basis.
More recently, a different kind of solar-powered process (the “STEP” method), involving less expensive materials, has been developed to carry out conversion of carbon dioxide to its component elements [Licht et al., J. Phys. Chem. Lett. 1, 2363-2368 (2010)]. However, this process does not involve transfer of electrons to CO2 from small-molecules, or even nanoparticles, but rather from a macroscopic metal electrode. In this case, carbon dioxide is initially dissolved in a molten lithium carbonate/lithium hydroxide salt, and its conversion to graphite and molecular oxygen occurs in an electrochemical cell operating at an elevated temperature of over 500° C.; at temperatures above 700° C. the main product is not graphite but carbon monoxide. This cell is solar-powered only in the sense that both the heat needed to maintain the high temperature, and the electricity used to drive the cell, can be derived from sunlight. That is, there is no intrinsic requirement for light in this process, which is fundamentally electrochemical and thermochemical, rather than photochemical. The use of solar energy in this process has the same challenging economic considerations as in the use of solar energy for other heating and electrical uses. Using other sources of energy (e.g. wind energy) might make more sense, not only from the point of view of economics but also in terms of maximizing the net carbon storage.
Yet another proposed method of using solar energy to chemically split CO2 involves the use of iron or cerium oxides in a redox cycle to initially reduce CO2 to CO, and then release O2 upon heating [Roeb et al. 2010, Science, 329. 773-774]. This process bears some similarity to the STEP method, in that the proposed use of solar energy is to drive a process that is not intrinsically photochemical. In this case, rather, the process is purely thermal, requiring temperatures of 800-2000° C. in order to release O2 from the oxide of iron or cerium. Thus, while the chemical reagent costs (e.g. for iron oxide) are extremely low, the capital costs required for thermal reactors capable of carrying out this high-temperature reaction, in a widely-distributed fashion over a large area of the Earth's surface, are likely to be formidable.